Dithering, also known as quantization error dispersion (QED), is a well known signal processing technique for improving the accuracy and resolution of analog to digital conversions. In general, the technique involves adding a dither signal to an analog signal prior to its conversion to a digital value in order to enhance the resolution of a particular A/D converter. After conversion, the dither signal is removed from the output of the conversion. The dither signal itself can be random white noise, a periodic ramp, square wave or triangle wave sweep, for example.
An analog to digital converter is limited in resolution according to the spacing between quantization levels. To illustrate, for a particular A/D converter, an analog signal falling between two adjacent quantization levels of the converter will always be converted to the lower digital value. Therefore, a quantization error is introduced into the output signal as different analog values falling within this interval generate the same digital value.
One solution is to increase the number of quantization levels but this solution is complicated and costly to implement. In addition, the digital conversion of signals having high dynamic range may be necessary and variations of low magnitude signals may generate the same digital value, i.e., lie between two quantization levels of the converter.
The addition of a dither signal, such as random white noise or a swept ramp function, to the input analog signal prior to digital conversion provides one way of compensating for these quantization errors. To illustrate, consider the case of an analog signal having a magnitude exactly half a quantization level. If no dither signal is applied, the converter will always output the lower digital value. If, however, a ramp signal having a magnitude of one quantization level is added to the input analog signal prior to conversion, the magnitude of the combined signal will remain below the threshold for half the time. As the ramp signal rises, the magnitude of the combined signal will rise above the next threshold for the other half of the time such that the converter will output the digital representation of the lower quantization level for half the time and the digital representation of the upper quantization level for the other half of the time. Digitally averaging the output of the converter over the ramp time yields a value corresponding to the average of the two quantization levels, which is the actual value of the input analog signal.
To further illustrate the technique of dithering, consider the case where the input analog signal is at a magnitude 10% above a particular quantization level, as opposed to 50% or half in the previous example. As the ramp dither signal is swept from low to high, the converter will output the digital value corresponding to the lower quantization level for the first 90% of the time and will output the digital value corresponding to the upper quantization level for the last 10% of the time. The average of the two digital outputs is the actual value of the analog input.
Numerous applications employ signal processing techniques that utilize dithering. Many of the applications that utilize dithering involve either choosing a particular dither signal and/or its method of injection into the input signal.
U.S. Pat. No. 5,189,418, issued to Bartz et al., teaches a technique for correcting in a dithered A/D conversion circuit. The dither signal is added to the analog input signal and then converted. The dither signal is removed by subtracting the digital dither value from the converted signal. A correction signal is also generated and subtracted from the analog input signal.
U.S. Pat. No. 5,187,481, issued to Hiller, teaches a circuit for A/D conversion whereby a dither signal is introduced before conversion and subtracted out of the resulting digital output. Gain control feedback loops are utilized to reduce gain error of the dither signal. The invention derives gain control feedback by performing correlation between the digital output and the dither signal.
U.S. Pat. No. 4,914,439, issued to Nakahashi et al., teaches a conversion system for digitizing an analog signal which adds a dither signal to the analog signal. After conversion of the combined signal, the dither signal is digitally subtracted from the converted signal. The dither signal is generated digitally and converted to analog by a D/A converter before being added to the input analog signal.
U.S. Pat. No. 5,451,947, issued to Morrison, discloses a system for A/D conversion which utilizes a dither signal that is modulated by another signal. The dither signal is modulated such that it is varied over at least one quantization interval of the A/D converter.
U.S. Pat. No. 5,416,481, issued to Chen, discloses a sigma-delta A/D converter that incorporates a dither circuit within the A/D converter package. The dither signal is added to the analog input signal so as to reduce the effects of quantization noise produced as a result of DC offset voltage in the analog signals.
U.S. Pat. No. 4,963,881, issued to Franceschini, teaches a method and apparatus for enhancing the resolution of an A/D converter. An analog dither signal is added to the input signal to be converted. Integration of the composite digital signal causes the effects of the dither signal to average to zero. The integration is performed using digital signal processing techniques to implement a DSPN correlator.
U.S. Pat. No. 4,700,173, issued to Araki et al., teaches using time division multiplexing techniques to multiplex an analog dither signal with a combined signal consisting of an analog dither signal added to the input signal. A single A/D converter is utilized to convert both the dither signal by itself and the dither signal combined with the input. The dither signal is then digitally subtracted from the converted signal.
Each of the references discussed above involve applying a dither signal to an analog to digital conversion system. In these devices, the dither signal is added to the analog input before conversion. Subsequent to conversion, the dither signal is digitally removed from the converted output signal thus reducing the effects of noise and distortion. Applications of the dithering technique include use in pulse code modulation (PCM) systems and compact disk players and other audio applications to reduce the distortion caused by quantization error noise. The dither signal is added to the analog input signal so as to turn the noise resulting from quantization errors into white noise which hardly effects the perceived quality of the reproduced sound.
A high level block diagram illustrating a prior art A/D conversion circuit where a dither signal is added to the analog input signal is shown in FIG. 1. The conversion circuit, generally referenced 12, comprises an adder or summer 14, A/D converter 16, dither signal generator 20 and a dither signal removal 18. The analog dither signal is added to the analog input signal before conversion to digital form. The combined signal is then converted via A/D converter 16 and the dither signal removed digitally to produce a digital output signal. As described in detail above, the addition of the dither signal to the input functions to improve the accuracy of the analog to digital conversion.
U.S. Pat. No. 4,994,803, issued to Blackham, teaches adding a digital pseudo random number dither signal to a digital input signal. The combined digital signal is then converted to analog, along with the dither signal. The analog counterpart of the digital random number dither signal is then subtracted from the analog output of the D/A converter. The introduction of the dither signal functions to reduce distortion and improve linearity.
In addition, U.S. Pat. No. 3,656,152, issued to Gundersen and U.S. Pat. No. 4,187,466, issued to Kasson et al., both teach conversion circuits that add an analog dither signal to an analog input signal before A/D conversion in addition to performing analog removal of the dither signal.